In recent years, regarding liquid fuels such as gasoline and gas oil, the control of a sulfur content and an aromatic hydrocarbon content has become rapidly stringent. Therefore, it has become essential to produce clean liquid fuels having low sulfur and aromatic hydrocarbon contents and being so-called friendly to the environment. Examples of the method for producing such clean fuels include a method in which hydrocarbons are synthesized by utilizing the so-called Fischer-Tropsch synthesis reaction (hereinafter, sometimes called as “FT synthesis reaction”) reducing a carbon monoxide with a molecular hydrogen (hereinafter, sometimes called as “FT synthesis method”), using a synthesis gas obtained by reforming a hydrocarbon such as a natural gas, that is, a mixed gas of carbon monoxide and molecular hydrogen (hydrogen gas), as a feedstock. By the FT synthesis method, not only can be produced liquid fuel base stocks rich in paraffin hydrocarbons and containing no sulfur component, but also can be produced a wax simultaneously. Then, the wax can be converted into a middle distillate (a fuel base stock such as kerosene and gas oil) by hydrocracking.
As the catalyst used for the FT synthesis method (hereinafter, sometimes called as “FT synthesis catalyst”), a catalyst in which an active metal such as iron, cobalt, and ruthenium is supported on a porous inorganic carrier such as silica or alumina is generally used (for example, see Patent Literature 1). In addition, with respect to the FT synthesis catalyst, it is reported that by using the above active metal in combination with a second component metal compound, the catalyst performance is enhanced (for example, see Patent Literature 2 and Patent Literature 3). Examples of the second component metal include sodium, magnesium, lithium, zirconium, and hafnium, which are used as required according to the object such as enhancing the conversion of carbon monoxide and increasing the chain growth probability, an indicator of an amount of wax generation.
As the FT synthesis reaction is remarkably exothermic and has a high reaction rate, it is considered that the reaction is completed in the proximity of the outer surface of the catalyst. Accordingly, when a certain amount of an active metal is loaded on a catalyst, the presence of the active metal existed in the proximity of the outer surface of a catalyst particle leads to a high activity of the catalyst, so that it is attempted to load the active metal to be in the proximity of the outer surface (for example, see Patent Literature 3).